It is one of the most important problems in aviation to improve the flying safety and minimize the effects of weather conditions on a flying mission.
Icing affects the aerodynamic properties of aircraft and may damage parts of their engines. Icing further affects visibility and radio communication, increases vibration and loads borne by structural components of aircraft. It is therefore absolutely necessary for pilots to be able to detect the first traces of icing and have qualitative information on the rate of icing so as to make the most effective use of the aircraft's anti-icing system. Such information is provided by ice detectors which indicate the severity of icing.
There are known numerous ice warning methods and devices.
Quite common are ice detectors of the pneumatic type. Their principle of operation is based on measuring the dynamic air pressure (cf. O.K. Trunov, "Obledeneniye samolyotov i sredstva borby s nim" /"Icing of Aircraft and Anti-Icing Systems"/, Machinostroyeniye Publishers, Moscow, 1965; cf. MK-8 device developed by "Canadian Applied Research Ltd."). The housing of such an ice detector has small holes in it. When icing occurs, the ice plugs the holes. As a result, the pressure inside the ice detector drops, and a pressure-sensitive element produces an "icing" signal.
The ice detector under review is simple and effective enough, which is not the case with the method it is intended to carry out. There may be an infinite variety of icing conditions. There may be situations when the holes are not plugged with ice (this is the case, for instance, with what is known as "horn-shaped" ice). Besides, the holes may be clogged with dust or some foreign matter, whereby the ice detector is rendered inoperative.
There are known ice detectors of the mechanical type, usually constructed in the form of scrapers, discs or rollers. Icing changes the path of motion of such devices, whereby a warning device is triggered off. A good case in point is the ice warning device manufactured by "D. Napier and Son Ltd." of Great Britain. Heavy icing normally renders such devices inoperative. Low sensitivity is another disadvantage of such devices. Finally, a warning signal is produced only when the crust of ice reaches a certain thickness.
Another type of ice detector makes use of the absorption of radioactive radiation by ice (cf. O.K. Trunov, "Obledeneniye samolyotov i sredstva borby s nim" /"Icing of Aircraft and Anti-Icing Systems"/, Machinostroyeniye Publishers, Moscow, 1965; cf. the type of ice detector developed by United Control Corporation of the United States). If a radiation source is covered with a crust of ice, the radioactive radiation is partially absorbed by that crust. The decrease in the intensity of the radiation flux is sensed by a warning device which produces an "icing" signal. Ice detectors of this type are too complicated in design and produce a warning signal only when the crust of ice is of a considerable thickness. Besides, the sensitivity of such devices is affected by the general radioactive background of space.
Some ice detectors are based upon measuring the capacitance between ice-covered electrodes. However, such devices are not reliable enough, and their sensitivity lacks stability.
Still another type of ice detector is based on the electric conductivity principle and senses the presence of moisture between electrodes. The presence of moisture reduces the resistance between the electrodes and accordingly increases the electric conductivity (cf. the device developed by "Rosemount Engineering Company" of the United States). The increase in the electric conductivity is sensed by an electric circuit which closes the contacts of a relay. But due to the effects of the air flow, such devices often produce unstable intermittent signals. Furthermore, they operate with a certain time lag and are hard to manufacture.
None of the above-mentioned ice detectors makes it possible to measure the rate of icing.
Electrothermal ice detectors provide a solution to the problem (cf. the device developed by "Teddington"). Such detectors sense conditions under which icing may take place. A device of this type comprises two heaters one of which is hit directly by the air flow. The other is behind the first one and protected from moisture. At subzero temperatures, supercooled moisture contained in the air flow makes the first heater cooler than the second. In order to maintain an equal temperature of both heaters, the first has to consume more power than the second. The difference in the amount of power consumed by the first and second heaters, respectively, is proportional to the amount of moisture evaporated by the first heater per unit of time and is indicative of the rate of icing.
One of the basic components of the device under review is a screen with a system of slots. These slots may get clogged with dust and mud and, in tropical areas, with insects, which affects the sensitivity or even causes a total failure of the detector. With an aircraft parked in the open at a subzero temperature, moisture may get between the slots and freeze and thus render the detector inoperative.
There is known a method for detecting icing of objects found in an air flow, according to which a zone of precipitation of supercooled droplets of water and a zone protected from precipitation of supercooled droplets of water, wherein natural turbulization of the air flow takes place, are simultaneously produced in the air flow. The temperature difference in these zones is then measured and is indicative of the icing conditions (cf. USSR Inventor's Certificate No. 154,064).
There is known a device for effecting the foregoing method for detecting icing of objects found in an air flow. The device comprises an ice detector which finds itself in an air flow. The detector's housing is so mounted on a flying object that one of its two working surfaces faces the air flow and produces a zone of precipitation of supercooled droplets of water. The second working surface is arranged opposite to the first working surface and produces a zone protected from precipitation of supercooled droplets of water, wherein natural turbulization of the air flow takes place. Each of the working surfaces carries a thermoelement connected to a respective input of a unit for measuring the difference of electric signals, which difference is indicative of the icing conditions (cf. USSR Inventor's Certificate No. 201,087).
In this device, the working surfaces are composed of strip thermocouples. The incoming air flow and moisture contained therein cool the hot junctions of the strip thermocouples. As a result, at the output of the unit for measuring the difference of electric signals there is produced a signal proportional to the rate of icing.
The above method and device for detecting icing of objects found in an air flow are such that reliable information on the onset of icing and the rate of icing is provided only at a constant pressure and velocity of the air flow. Naturally, the confidence of such information is always less than 100 percent in actual flying conditions. Besides, the device under review is complicated in design and consumes too much power (up to 1,000 wt.). In an attempt to raise the accuracy of measurements, the working surfaces of the device are heated; the heating elements are insulated by layers of an insulating material. When in operation, the temperature of the heating elements reaches 350.degree. C. Due to different thermal coefficients of volume expansion, the monolithic structure of the insulating materials deteriorates in the course of operation; this deterioration and microcracking, as well as the direct contact between the junctions of the strip thermocouples and the air flow all tend to reduce the service life of the ice detector.